Noise has always been an undesirable characteristic of switched reluctance motors in that such motors tend to produce a noticeable whine or hum during operation. The noise is believed to be generated by vibration of the stator as it is deformed during each phase energization and deenergization. The stator of a switched reluctance motors is typically comprised of a ring having a plurality of inwardly extending stator poles. During each phase energization, opposing stator poles are electrically energized to form a magnetic field that draws the nearest rotor poles into alignment therewith. The magnetic field that draws the rotor into alignment with the stators may also deform the stator by drawing the opposing energized stator poles towards the rotor. These attractive forces tend to distort the stator ring from its generally circular configuration. Stated another way, the opposing attractive forces tend to "ovalize" the stator as opposing, energized poles are drawn toward each other. Upon de-energization of the stator poles, the stator "springs" or "snaps" back to its original circular configuration. It will of course be appreciated that distortion of the stator pole is extremely slight. Nevertheless, the distortion produced during each phase energization and de-energization causes the entire motor to "vibrate" or "hum" during continuous operation thereby, producing the undesirable acoustical noise.
U.S. patent application Ser. No. 08/545,085 to Pengov discloses a two-phase switched reluctance motor that, among other things, addresses the noise problems of two-phase motors. The disclosed motor has a rotor with wide rotor poles and narrow rotor poles. During each phase energization, the rotor is sequentially advanced in a two-step fashion. In the 8/4 switched reluctance motor shown in the '085 application, during the first step, the leading edges of the wide rotor poles interact with a first pair of opposed energized stator poles. During the second step, narrow rotor poles are drawn into alignment with a second set of opposed stator poles. Importantly, because of the width of the wide rotor poles, the attractive forces exerted on this second set of opposed stator poles takes place while attractive forces are still exerted on the first set of opposed stator poles. Thus, during each phase energization, the stator first experiences opposed inward forces, 180.degree. apart, drawing the stator out of round, then during the second phase experience, a second set of forces, 180.degree. apart but 90.degree. to the first forces, drawing the sides of the stator inwardly. The original radial forces thus are offset by the second set of radial forces that are 90.degree. therefrom. As a result, during the second step or portion of each phase energization, the stator is generally stiffened by inward radial forces 90.degree. apart created by two sets of opposed stator poles. Upon deenergization of a phase, rather than switching from an oval configuration to a round configuration, the generally balanced stator relaxes more uniformly. The motor shown in U.S. patent application Ser. No. 08/545,085 thus, in addition to the other operating characteristics resulting from the unique rotor design, provides a less noisy two-phase switched reluctance motor.
The present invention provides an improvement to the motor disclosed in U.S. patent application Ser. No. 08/545,085 and provides a switched reluctance motor that axially distributes the internal attractive forces created between the stator and the rotor to reduce acoustical noise.